The present invention relates to measuring and holding the peak voltage of a signal in a high noise environment. It has been developed to aid the measurement of delivered energy and thoracic impedance of a patient during defibrillation. Since a defibrillator generates an extreme amount of noise during operation, the requirements for a circuit to detect and hold a peak voltage, i.e. a peak detector, include high noise immunity as well as high precision. Heretofore known peak detectors do not adequately fulfill the high noise immunity requirement.
One type of known peak detector circuit is illustrated in FIG. 1. The peak voltage of an analog input signal V.sub.in is held in a capacitor C1. There is a voltage drop across a diode D1, which causes capacitor C1 to be undercharged. This limits the accuracy of the circuit. Additionally, the circuit operates over a very wide bandwidth so there is little noise immunity.
Another known peak detector is shown in FIG. 2. The peak value of input signal V.sub.in is held in capacitor C2. This circuit has a large amount of negative feedback which gives it high precision. Slew rate limiting can be achieved within first amplifier A1 to mitigate the effect of noise impulses; however in this case, large differential voltages appear across the input nodes of amplifier A1 which may cause the input transistors of amplifier A1 to saturate. A delay in switching off amplifier A1 due to the saturation will cause capacitor C2 to overcharge. This can result in a significant error in the measurement of the peak value of the input voltage.